Further studies on the substitutional behavior of 1,2-Mo2Br2(CH2SiMe3)4.: Alkyl, amide, phosphide, alkoxide, and thiolate for bromide exchange and isomerizations of 1,1-and 1,2-Mo2X2(CH2SiMe3)4 compounds

Reactions involving 1,2-Mo2Br2(CH2SiMe3)(4) and each of LiNMe2, and HNMe2 in hexane at -50 degrees C proceed to give 1,1'-Mo-2(NMe2)BrR4, where R = CH2SiMe3, as the first product of Br for amide exchange at the Mo-2(6+) center. Further substitution in reactions with LiNMe2 gives 1,1-Mo-2(NMe2)(2)R-4, but with HNMe2 the 1,2-Mo-2(NMe2)(2)R-4 isomer is formed. These two do not interconvert thermally (> 100 degrees C) and undergo aminolysis (HNMe2-d(6)) and alcoholysis ((BuOH)-Bu-t) with retention of regiochemistry. 1,1- to 1,2-isomerization of Mo-2(NMe2)(2)R-4 is catalyzed by the presence of Me2NH2Br. The mechanisms of these metathetic reactions are discussed in terms of plausible pathways that may promote alkyl group migration across the Mo=Mo bond. Further studies of the bromide substitution by alkyl, aryl, alkoxide, thiolate, and phosphide ligands are described for 1,1'-Mo-2(NMe2)BrR4 and 1,2-Mo2Br2R4. The new compounds of formula Mo-2(NMe2)YR4 (Y = CH2SiMe3, Ph, (OBu)-Bu-t, (OPr)-Pr-t, (SBu)-Bu-t, PPh2, and Br) and Mo-2((SBu)-Bu-t)(2)R-4 have been characterized by variable-temperature H-1 and C-13{H-1} NMR spectroscopy and mass spectrometry and for Y = PPh2 by single-crystal X-ray crystallography.